Sir Michael Lyons has hit back against calls for the BBC Trust to be scrapped, a pledge the Conservative Party has made if it comes to power, arguing that the result could be the creation of a "glorified complaints office".

Lyons, the BBC Trust chairman, speaking to the Manchester Statistical Society today, also said he saw it as the trust's mission to stamp out an "imperial compulsion" that has resided in the corporation since it enjoyed monopoly UK broadcasting status in the days of the first director general, John Reith, in the 1920s.

"But, that said, a more compact BBC will undoubtedly mean making some tough choices," he said. "The BBC needs to concentrate on its important and widely valued public role rather than seeking to become an international communications company."

Lyons added that many of the achievements of the often-criticised BBC Trust, the corporation's regulatory and governance body, have been overlooked and that working closely with the director general, Mark Thompson, and the executive board has proved to be beneficial, not detrimental, as some have argued.

"Being part of the BBC keeps us close to the coalface [and] I believe strongly that having the trust as part of the BBC is a strength not a weakness," he said. "When we see things going wrong we can act quickly and decisively to put this right. The public would be short changed if the trust were replaced by a glorified complaints office."

Lyons added that the BBC Trust is not conflicted in a role that critics characterise as being both cheerleader and regulator for the corporation. "What the trust is not is the BBC's regulator. That's the job of Ofcom," he said.

"Our job is to steer that tricky course between independence and accountability," Lyons added, referring to the BBC Trust as akin to a supervisory board.

However, Lyons today defended the trust, insisting it was capable of reining in the BBC's expansionist tendencies.

"There is a view of the BBC that there is within its DNA a kind of imperial compulsion," he said, explaining why the BBC needs "clear boundaries" to emerge from the current strategic review.

"According to this view, the BBC is driven by an insatiable desire to expand, to colonise, to establish its forces in every far-flung corner of broadcasting and publishing. That is not something the trust, as the representative of the public, will allow to happen."

Lyons added that the public impact of an expansionist BBC was a reduction of choice in the market that ultimately was detrimental for consumers. "You, the public, would be the losers," he said. "The trust is clear that the BBC must be a good corporate neighbour to others in the media marketplace."

He admitted that the BBC can seem to be a "pretty big and insensitive presence in the marketplace". But he warned that when some rivals claim to be threatened, such as BSkyB, with its "colossal scale, ambition and financial muscle", "it can be hard to take such charges entirely seriously".

"We have no issue at all with the BBC competing ferociously where it matters, on quality, but the trust has no wish to see the BBC reassume its monopoly position," he said.

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